The dominant role of structure for solute transport in soil: experimental evidence and modelling of structure and transport in a field experiment

摘要

A classical transport experiment was performed in a field plot of 2.5 m2using the dye tracer brilliant blue. The measured tracer distributiondemonstrates the dominant role of the heterogeneous soil structure for solutetransport. As with many other published experiments, this evidences the needof considering the macroscopic structure of soil to predict flow andtransport.We combine three different approaches to represent the relevant structure ofthe specific situation of our experiment: i) direct measurement, ii)statistical description of heterogeneities and iii) a conceptual model ofstructure formation. The structure of soil layers was directly obtained fromserial sections in the field. The sub-scale heterogeneity within the soilhorizons was modelled through correlated random fields with estimatedcorrelation lengths and anisotropy. Earthworm burrows played a dominant roleat the transition between the upper soil horizon and the subsoil. A modelbased on percolation theory is introduced that mimics the geometry ofearthworm burrow systems.The hydraulic material properties of the different structural units wereobtained by direct measurements where available and by a best estimateotherwise. From the hydraulic structure, the 3-dimensional velocity field ofwater was calculated by solving Richards' Equation and solute transport wassimulated. The simulated tracer distribution compares reasonably well withthe experimental data. We conclude that a rough representation of thestructure and a rough representation of the hydraulic properties might besufficient to predict flow and transport, but both elements are definitelyrequired.